The preparation of polyurethane and polyurethane/urea elastomers and adhesives traditionally requires the combination of isocyanates or isocyanate containing resins with crosslinking agents, e.g., polyamines or polyols, in precise ratios. That is because the reactivity of the combined isocyanates and isocyanate terminated prepolymers and the properties of the final polyurethane and polyurethane/urea vulcanizates are dependent upon the curative-resin ratio or stoichiometry. It is further recognized in the preparation of the polyurethane/urea elastomers that many of the presently used aromatic amine curatives have potential health and safety problems are crystalline solids which must be melted at high temperature and used in a molten state. A final problem in preparing polyurethane/urea elastomers is that the limited working time ("pot-life") available complicates the production of multiple castings and large parts that require large amounts of polyurethane/urea.
A number of approaches have been taken to overcome such problems as those mentioned above. These rely upon (a) the inhibition of the isocyanate, (b) the retardation of the curative, and (c) the blocking of catalysts which promote crosslinker-isocyanate reactions. Examples of the first approach (a), include the use of reaction products of isocyanates with oximes, phenols, or caprolactams. These products are stable at room temperature but revert to the starting components at higher temperatures. Examples of the second approach (b) include deactivated complexes of aromatic amines which form stable admixtures with isocyanate containing materials at room temperature. One commercially available material sold under the trademark, Caytur.RTM. 21, is the 2.5:1 complex of methylenedianiline (MDA) with sodium chloride suspended in a carrier fluid. Heat destroys the weak complex and the liberated amine crosslinks the isocyanate containing material. An example of the third approach is a blend of a curative and an isocyanate which undergo a slow crosslinking reaction in the absence of a catalyst. A delayed catalyst, such as a thioalkanoic-tin complex, can be added. The three component blend has an extended working life at room temperature but rapidly cures upon heating.
The preparation of polyisocyanurate containing networks formed by the trimerization and chain extension of isocyanate terminated prepolymer in the presence of polyols is known. In this method the isocyanate group are caused to react to form the polyisocyanurate and this reaction can be effected through catalyst systems such as quaternary ammonium salts. The polyisocyanurate containing vulcanizates or networks are adapted for use in producing molded parts, laminating and impregnating applications as well as adhesives, coatings and so forth. Representative patents which disclose the trimerization of polyisocyanate resins are as follows:
U.S. Pat. No. 4,880,845 discloses the catalytic trimerization of aromatic isocyanate terminated prepolymers in the presence of short chain and long chain diols, e.g., butane diol and polyether polyols to rapidly produce polyisocyanurate and polyurethane containing products. Trimerization catalysts include the organic acid salts of 1,8-diaza-bicyclo(5,4,0)undec-7-ene. Ortho-carboxylic acid esters used as a catalyst component include ortho-carbonic acid tetraethylester, ortho-formic acid triethylester and the like. Controlled induction periods of a few seconds are noted.
U.S. Pat. No. 4,126,742 discloses a process for producing polyisocyanurate elastomers which comprises polymerizing aromatic polyisocyanates, e.g., methylene diisocyanate, with a small amount of polyol in the presence of a trimerization catalyst. Examples of trimerizing catalysts include tertiary amines such as N,N-dialkyl piperazine; 1,4-diazabicyclo[2.2.2.]octane and many more.
U.S. Pat. No. 4,033,908 discloses the preparation of polyisocyanurate foams by reacting a polyisocyanate in the presence of a trimerization catalyst and a blowing agent. Aromatic polyisocyanates such as polymethylenepolyphenylpolyisocyanate are trimerized in the presence of organic cyclic carbonates and liquid alkylene carbonates, tertiary amine metal salts as trimer catalysts.
U.S. Pat. No. 4,855,383 discloses a storage-stable, liquid composition comprising an isocyanate functional compound, an epoxy component, an alkylating agent and a tertiary amine catalyst precursor. The tertiary amine precursor forms a quaternary ammonium salt catalyst in situ for achieving crosslinking of the isocyanate via trimerization. In practice an isocyanate-functional prepolymer, either derived from aromatic or aliphatic polyisocyanate, is mixed with the epoxy component and alkylating agent. In the presence of a quaternary ammonium catalyst the isocyanate is trimerized to form a polyisocyanurate resin.
U.S. Pat. No. 5,102,918 discloses a modified organic polyisocyanate having an isocyanurate ring. The modified polyisocyanate is prepared from an aromatic polyisocyanate, such as toluenediisocyanate or diphenylmethanediisocyanate or isocyanate terminated prepolymers where the polyol is the polyether or polyester polyol. Catalysts used for effecting isocyanuration of the isocyanates include amines such as 2,4-bis(dimethylaminomethyl)pheny, N,N',N",-tris(dimethylaminopropyl)hexahydrotriazine and diazabicycloundecene.
U.S. Pat. No. 4,602,049 discloses the use of amidinium salts as a catalyst for converting isocyanates to products having isocyanurate and carbodiimide linkages. In the examples, polyol was added with the amidinium catalyst for effecting isocyanurate and carbodiimide linkages in the resulting product.